1. Field of the Invention
The invention relates to an integrated circuit having a hydrogen barrier layer to protect elements containing ferroelectric or high-dielectric constant metal oxide materials.
2. Description of the Related Art
Ferroelectric compounds possess favorable characteristics for use in nonvolatile integrated circuit memories. See Miller, U.S. Pat. No. 5,046,043. A ferroelectric device, such as a capacitor, is useful as a nonvolatile memory when it possess desired electronic characteristics, such as high residual polarization, good coercive field, high fatigue resistance, and low leakage current. Lead-containing ABO3-type ferroelectric oxides such as PZT (lead titanate zirconate) and PLZT (lanthanum lead titanate zirconate) have been studied for practical use in integrated circuits. Layered superlattice material oxides have also been studied for use in integrated circuits. See Watanabe, U.S. Pat. No. 5,434,102. Layered superlattice material compounds exhibit characteristics in ferroelectric memories that are orders of magnitude superior to those of PZT and PLZT compounds. Integrated circuit devices containing ferroelectric elements are currently being manufactured. Nevertheless, the persistent problem of hydrogen degradation during the manufacturing process hinders the economical production in commercial quantities of ferroelectric memories and other IC devices using the layered superlattice material compounds with the desired electronic characteristics.
A typical ferroelectric memory device in an integrated circuit contains a semiconductor substrate and a metal-oxide semiconductor field-effect transistor (MOSFET) in electrical contact with a ferroelectric device, usually a ferroelectric capacitor. A ferroelectric capacitor typically contains a thin film containing ferroelectric metal oxide located between a first, bottom electrode and a second, top electrode, the electrodes typically containing platinum. During manufacture of the circuit, the MOSFET is subjected to conditions causing defects in the silicon substrate. For example, the CMOS/MOSFET manufacturing process usually includes high energy steps, such as ion-mill etching and plasma etching. Defects also arise during heat treatment for crystallization of the ferroelectric thin film at relatively high temperatures, often in the range 500xc2x0 C. to 900xc2x0 C. As a result, numerous defects are generated in the single crystal structure of the semiconductor silicon substrate, leading to deterioration in the electronic characteristics of the MOSFET.
To restore the silicon properties of the MOSFET/CMOS, the manufacturing process typically includes a hydrogen annealing step in which defects, such as dangling bonds, are eliminated by utilizing the reducing property of hydrogen. Various techniques have been developed to effect the hydrogen annealing, such as a forming gas anneal (xe2x80x9cFGAxe2x80x9d). Conventionally, FGA treatments are conducted under ambient conditions in a H2xe2x80x94N2 gas mixture between 350xc2x0 C. and 550xc2x0 C., typically around 400xc2x0 C. to 450xc2x0 C., for a time period of about 30 minutes. In addition, the CMOS/MOSFET manufacturing process requires other fabrication steps that expose the integrated circuit to hydrogen, often at elevated temperatures, such as hydrogen-rich plasma CVD processes for depositing metals and dielectrics, growth of silicon dioxide from silane or TEOS sources, and etching processes using hydrogen and hydrogen plasma. During processes that involve hydrogen, the hydrogen diffuses principally through the top electrode to the ferroelectric thin film, but also from the side edges of the capacitor, and reduces the oxides contained in the ferroelectric material. The absorbed hydrogen also metallizes the surface of the ferroelectric thin film by reducing metal oxides. As a result of these effects, the electronic properties of the capacitor are degraded. After the forming-gas anneal (FGA), the remnant polarization of the ferroelectrics is very low and no longer suitable for storing information. An increase in leakage currents also results. In addition, the adhesivity of the ferroelectric thin film to the upper electrode is lowered by the chemical change taking place at the interface. Alternatively, the upper electrode is pushed up by the oxygen gas, water, and other products of the oxidation-reduction reactions taking place. Thus, peeling is likely to take place at the interface between the top electrode and the ferroelectric thin film. In addition, hydrogen also can reach the lower electrode, leading to internal stresses that cause the capacitor to peel off its substrate. These problems are acute in ferroelectric memories containing layered superlattice material compounds because these oxide compounds are particularly complex and prone to degradation by hydrogen-reduction.
A related problem encountered in the fabrication of ferroelectric devices is the stress arising in and between the different circuit layers as a result of the manufacturing processes. The ferroelectric compounds comprise metal oxides. The products of the hydrogen reduction reactions cause an increase in the total volume of the ferroelectric element. As a result, the ferroelectric thin film exerts an upward pressure on the layers above it.
Several methods have been reported in the art to inhibit or reverse hydrogen degradation of desired electronic properties in ferroelectric oxide materials. Oxygen-recovery annealing at high temperature (800xc2x0 C.) for about one hour results in virtually complete recovery of the ferroelectric properties degraded by hydrogen treatments; but the high-temperature oxygen-anneal itself may generate defects in silicon crystalline structure, and it may offset somewhat the positive effects of any prior forming-gas anneal on the CMOS characteristics. Also, if hydrogen reactions have caused structural damage to the ferroelectric device, such as peeling, then a recovery anneal is not able to reverse effectively the damage.
To reduce the detrimental effects of the hydrogen heat treatment and protect the ferroelectric metal oxide element, the prior art also teaches the application of hydrogen barrier layers to inhibit the diffusion of hydrogen into the ferroelectric material. The barrier layer is typically located over the ferroelectric element, but it can also be located below and laterally to the sides of the element.
Hydrogen degradation is also a problem in complex metal oxides used in nonferroelectric, high-dielectric constant applications in integrated circuits. Hydrogen reactions cause structural damage, as described above for ferroelectric oxides, and cause degradation of dielectric properties. Examples of metal oxides subject to hydrogen degradation include barium strontium titanate (xe2x80x9cBSTxe2x80x9d), barium strontium niobate (xe2x80x9cBSNxe2x80x9d), certain ABO3-type perovskites, and certain layered superlattice materials. Hydrogen barrier layers are, therefore, used also to protect nonferroelectric, high-dielectric constant metal oxides.
It is known in the art to use a hydrogen barrier layer comprising a nitride of aluminum, silicon or titanium, that is, AlN, Si3N4, or Ti3N4.
Typically, hydrogen barrier layers known in the art are not completely effective in preventing hydrogen diffusion and the resulting hydrogen degradation of metal oxides. Thus, even when a diffusion barrier is used, it is not uncommon for structural damage to arise in the ferroelectric or dielectric device and for hydrogen to reach the metal oxide layer and degrade the desired ferroelectric or dielectric properties of the metal oxide material. Therefore, it would be useful to have new materials different from those known in the art to obtain the benefits of a hydrogen barrier layer in protecting ferroelectric and dielectric oxide materials, in particular, ferroelectric layered superlattice materials, from hydrogen degradation.
The invention solves the above problem by disclosing novel compositions of material for a hydrogen diffusion barrier.
A feature of the invention is a hydrogen barrier layer comprising a nitride of aluminum and another chemical element selected from the group consisting of silicon, titanium, tantalum, niobium, copper and tungsten. Such nitrides include: aluminum titanium nitride (Al2Ti3N6), aluminum silicon nitride (Al2Si3N6), aluminum niobium nitride (AlNb3N6), aluminum tantalum nitride (AlTa3N6), aluminum copper nitride (Al2Cu3N4). Another feature of the invention is a hydrogen barrier layer comprising a nitride of copper or tungsten. Such nitrides include tungsten nitride (WN) and copper nitride (CU3N2).
Another feature of the invention is an integrated circuit in which a hydrogen barrier layer as described above is located to inhibit diffusion of hydrogen to a thin film of metal oxide material. Preferably, the inventive hydrogen barrier layer is located directly over the thin film of metal oxide material, but it may also be located below or laterally to the sides of the thin film. The metal oxide material may be ferroelectric material, or it may be nonferroelectric, high-dielectric constant material. The composition of a thin film of ferroelectric material may be selected from a group of suitable ferroelectric oxide materials, including but not limited to: an ABO3-type perovskite, such as a titanate (e.g., BaTiO3, SrTiO3, PbTiO3, PbZrTiO3), a niobate (e.g., KNbO3), and, preferably, a layered superlattice compound. Alternatively, a thin film of nonferroelectric, high-dielectric constant materials may be selected from a group including but not limited to: barium strontium titanate (xe2x80x9cBSTxe2x80x9d), barium strontium niobate (xe2x80x9cBSNxe2x80x9d), certain ABO3-type perovskites, and certain layered superlattice materials.
Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.